1. Field of the Invention
The present invention relates to a signal detection RF coil which generates a magnetic resonance signal by applying RF pulses to an object to be examined in a static magnetic field and at the same time, acquires a magnetic resonance signal, and a magnetic resonance imaging apparatus which using the RF coil.
2. Description of the Related Art
A magnetic resonance imaging (MRI) apparatus is an apparatus which images the chemical and physical microscopic information of a substance by using a phenomenon in which when a group of nuclei having a unique magnetic moment is placed in a uniform static magnetic field, they resonantly absorb energy of an RF magnetic field that rotates at a specific frequency. Among recent techniques associated with such a magnetic resonance imaging apparatus, a phased array technique is available, in which a plurality of surface coils are arranged with respect to a region of interest, and an image with a high S/N ratio is acquired by receiving a magnetic resonance signal.
For example, a magnetic resonance imaging apparatus is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 4-42937, in which a plurality of surface coils (e.g., looped coils) are arranged in a desired region of an object to be examined which is to be imaged, and magnetic resonance signals from the object are detected through these surface coils, respectively. The detected magnetic resonance signals are converted into a plurality of series of image data by imaging processing. Data corresponding to the same spatial position are multiplied by predetermined weighting functions (function determined in advance on the basis of the distribution of RF magnetic fields generated by the respective surface coils), and the resultant data are added together. The respective pixel data obtained in this manner are combined to provide an image with a high S/N ratio of an overall desired region of the object.
A parallel imaging method (to be referred to as a “PI method” hereinafter) which is a high-speed imaging method using multiple surface coils is proposed in Magnetic Resonance in Medicine, Vol. 29, pp. 681 to 688 (1993) or Magnetic Resonance in Medicine, Vol. 30, pp. 142 to 145 (1993). The contents of the former are also disclosed in “Rapid MRI using multiple receivers producing multiple phase-encoded data derived from a single NMR response” (U.S. Pat. No. 4,857,846). The phased array technique is also introduced as a noteworthy technique in Magnetic Resonance in Medicine, Vol. 42, pp. 952-962 (1999). According to the techniques disclosed in these references, when a plurality of surface coils are arranged around a region of interest, the data amount of raw MRI data in the encoding direction can be reduced by almost the reciprocal of the number of coils arrayed in the direction. Assume that a 256×256 matrix axial image is to be acquired. In this case, if the X and Y directions correspond to the reading and encoding directions, respectively, 256 data are generally sampled while a gradient field is applied in the X direction. This operation is repeated 256 times while the gradient filed pulse intensity in the Y direction is changed in predetermined steps, thereby obtaining 256×256 raw data. By performing a Fourier transform of the raw data, an axial image can be obtained. Assume that two surface coils are so arranged as to sandwich the patient in the vertical direction, and the PI method is used. In this case, even if the number of times data acquisition is done while the gradient field pulse intensity in the Y direction is changed in predetermined steps, a 256×256 matrix image can be reproduced properly.
In this manner, a data acquisition time T is reduced to 1/n, and the S/N ratio is reduced to 1/n1/2. By acquiring data using a plurality of surface coils with a high S/N ratio, a decrease in S/N ration due to a decrease in data acquisition time can be compensated for. In addition, if surface coils are arrayed in the X direction or Z direction (static magnetic field direction), the number of times of encoding can be decreased in accordance with the number of coils arrayed in encoding in the X or Z direction. This makes it possible to shorten the data acquisition time. That is, high-speed imaging can be done.
However, conventional magnetic resonance imaging apparatuses are not designed to arrange RF coils in the three directions, i.e., the X, Y, and Z directions, but are designed to arrange RF coils in the two directions, i.e., the X and Y directions or the Y and Z directions. When, therefore, an abdominal region is to be imaged by using the PI method, the number of times of encoding in the Z or X direction cannot be decreased. Furthermore, when a slice in an arbitrary direction (oblique imaging) is selected, the PI method is difficult to apply.